1. Field of the Invention
This invention relates generally to methods and apparatus for transforming energy from a heat source into a useable form using a working fluid that is expanded and regenerated. This invention further relates to a method and apparatus for improving the heat utilization efficiency of a thermodynamic cycle.
2. Brief Description of the Background Art
In the Rankine cycle, a working fluid such as water, ammonia or freon is evaporated in an evaporator utilizing an available heat source. The evaporated gaseous working fluid is expanded across a turbine to transform its energy into useable form. The spent gaseous working fluid is then condensed in a condenser using an available cooling medium. The pressure of the condensed working medium is increased by pumping, followed by evaporation and so on to continue the cycle.
The Exergy cycle, described in U.S. Pat. No. 4,346,561, utilizes a binary or multi-component working fluid. This cycle operates generally on the principle that a binary working fluid is pumped as a liquid to a high working pressure and is heated to partially vaporize the working fluid. The fluid is then flashed to separate high and low boiling working fluids. The low boiling component is expanded through a turbine, to drive the turbine, while the high boiling component has heat recovered for use in heating the binary working fluid prior to evaporation. The high boiling component is then mixed with the spent low boiling working fluid to absorb the spent working fluid in a condenser in the presence of a cooling medium.
A theoretical comparison of the conventional Rankine cycle and the Exergy cycle demonstrates the improved efficiency of the new cycle over the Rankine cycle when an available, relatively low temperature heat source such as ocean water, geothermal energy or the like is employed.
In applicant's further invention referred to as the Basic Kalina cycle, the subject of U.S. Pat. No. 4,489,563, relatively lower temperature available heat is utilized to effect partial distillation of at least a portion of a multi-component fluid stream at an intermediate pressure to generate working fluid fractions of different compositions. The fractions are used to produce at least one main rich solution which is relatively enriched with respect to the lower boiling component, and to produce one lean solution which is relatively impoverished with respect to the lower boiling component. The pressure of the main rich solution is increased; thereafter, it is evaporated to produce a charged gaseous main working fluid. The main working fluid is expanded to a low pressure level to convert energy to useable form. The spent low pressure level working fluid is condensed in a main absorption stage by dissolving with cooling in the lean solution to regenerate an initial working fluid for reuse.
In any process of converting thermal energy to a useable form, a major loss of available energy in the heat source occurs in the process of boiling or evaporating the working fluid. This loss of available energy (known as exergy or essergy) is due to the mismatch of the enthalpy-temperature characteristics of the heat source and the working fluid in the boiler. Simply put, for any given enthalpy the temperature of the heat source is always greater than the temperature of the working fluid. Ideally, this temperature difference would be almost, but not quite, zero. This mismatch occurs both in the classical Rankine cycle, using a pure substance as a working fluid, as well as in the Kalina and Exergy cycles described above, using a mixture as a working fluid. The use of a mixture as a working fluid in the manner of the Kalina and Exergy cycles reduces these losses to a significant extent. However, it would be highly desireable to further reduce these losses in any cycle.
In the conventional Rankine cycle the losses arising from mismatching of the enthalpy-temperature characteristics of the heat source and the working fluid constitute about 25% of the available energy. With a cycle such as that described in U.S. Pat. No. 4,489,563, the loss of exergy in the boiler due to enthalpy-temperature characteristics mismatching would constitute about 14% of all of the available exergy.
The overall boiling process in a thermodynamic cycle can be viewed for discussion purposes as consisting of three distinct parts: preheating, evaporation and superheating. The quantity of heat in the temperature range suitable for superheating is generally much greater than necessary, or the quantity of heat in the temperature range suitable for evaporation is much smaller than necessary. A portion of the high temperature heat which would be suitable for high temperature superheating is used for evaporation in conventional processes. This causes very large temperature differences between the two streams, and as a result, irreversible losses of exergy.
In accordance with another invention of the applicant, the subject of U.S. Pat. No. 4,604,867, a fluid may be diverted to a reheater after initial expansion in the turbine to increase the temperature available for superheating. After return to the turbine, and additional expansion, the fluid is withdrawn from the turbine and cooled in an intercooler. Afterwards, the fluid is returned to the turbine for additional expansion. The cooling of the turbine gas may provide additional heat for evaporation. Intercooling provides compensation for the heat used in reheating and may provide recuperation of heat available which would otherwise remain unused following final turbine expansion.
In the past preheating of a working fluid is usually performed by extraction of part of the working fluid stream between turbine stages. This is followed by injection of the extracted stream or streams into the stream of feed water to the turbine. As a result heat of a lower temperature level may perform preheating, which occurs at relatively low temperature levels. Therefore, in general, this process increases the efficiency of the power plant.
However, conventional preheating has a drawback, because the steam used for preheating has a temperature which is significantly higher than the temperature of the feed water into which it is injected. This steam may even have a temperature which is higher than the temperature of the feed water obtained after injection. This creates irreversibilities and lowers the potential efficiency of the power plant.
It would be highly desirable to provide a process and apparatus which avoids the creation of these irreversibilities and thereby increases the efficiency of the power plant.